1. Field of the Invention
This invention relates to a method for preparing a stable silica sol as well as to a material-saving, non-polluting, non-flammable inorganic coating composition which is characterized by containing said silica sol as binder and by early developing of the strength of the coating at room temperature.
2. Description of the Prior Art Products of silica sol which have silica particles having silicate framework as principal ingredient in an amount of 20 - 31% are commercially available at present. These products are frozen at or below 0.degree. C and cannot revert to sols, even after having been thawed, to be unusable. Because of these drawbacks in stability, especially at lower temperatures, their application is limited. Any solution for these problems has been desired.
Various methods are proposed to stabilize a silica sol. As a method for stabilizing by addition of a chemical it is well known to use a volatile nitrogen-containing base having a low molecular weight, such as an organic amine and a quaternary ammonium hydroxide. However, these chemicals increase the pH of a silica sol due to their high basicity. In addition, when the temperature of the sol rises or the water in the sol decreases by evaporation during storage, the added nitrogenous base is concurrently removed due to its volatility and the silica sol is concentrated. As a result, the sol converts to a gel and silica crystals precipitate. These are important drawbacks of said method. Further, a method for stabilizing a silica sol where a water-soluble organic liquid, such as ethylene glycol, is added to a sol has been also known. However, this stabilizing method has a drawback that the resulted sol gels on cooling.
On the other hand, uses of silica sols as coating material have been proposed. For example, when the coating is desired to be reinforced and improved in the case of coating a surface of an inorganic or organic material with a silica sol, methanol is added to an aqueous dispersion of silica particules having a definite surface area. The amount of methanol to be added is within a defined range of molar ratio of methanol to water. Thus-formed silica sol composition is applied onto the surface to form a reinforced coating. In some cases, methanol retained in the coating is burnt immediately after applying to promote forming of improved film. However, the coatings according to these methods have low strengths, and the control of burning operation is difficult to practice.
As method for stabilizing a silica sol is well known a process where an anionic surface active agent is employed to form a colloidal silica. The prepared colloidal silica is mixed with a polyvinyl acetate emulsion. The mixture is used to treat a sheet for photosensitive paper. Thus, the mixture is not for using as binder of coating material. Compositions where a synthetic resin emulsion has been added to a silica sol to increase the binder force are known as binder for coating material. When a water-based coating material containing, as binder, silica hydrosol added with a synthetic resin emulsion is employed, a large amount of said synthetic resin emulsion is required to add, especially in the case of a reinforced coating of 2 mm or more in thickness being desired. In order to prevent the deposition of aggregate and cracking of coating in the course of drying, bentonite is added, together with a large amount of a synthetic resin emulsion. However, this method has a drawback that the resulted coating has a lowered water resistance and a low strength due to the large amount added of bentonite.